How to choose between different flat roof material types

Flat roofs are harder to waterproof than shingle roofs, so it's important to make sure it's done right! In this article we're going to go over a few categories of materials applied on flat roofs and help you decide what's right for your roof.

Modified Bitumen (torch down)

Modified Bitumen, also commonly called torch down, is the most commonly installed roofing system in the United States. The material is asphalt modified with polymers (plastics) to make it durable and flexible.

It consists of a base sheet installed on top of the roof deck, cap sheet on top, and sometimes a third layer between the 2. The third layer is often called the smooth layer, because it is not granulated like the cap layer. The smooth layer is recommended to be installed on any living space. It is also required by code to be installed on roofs that are completely flat. Some flat roofs have a small amount of slope, which prevents water from pooling.

Most manufacturers offer 10 year warranties, which are extended to 15 years if the smooth layer is used. The typical cost of a modified roofing system is $450-$800/square, with the smooth layer adding $100-$200/square. Like all roofs, prices vary significantly by region—check out our roof cost guide for more details on regional pricing.

Synthetic Membranes

Synthetic membranes, also called, single-ply membrane are thin, flexible sheets made from synthetic materials. Common types include EPDM, TPO, and PVC, each offering strong waterproofing and resistance to weather and UV rays.

Membranes are a usually bit more expensive than modified bitumen, but have longer lifespans. Warranties range from 15 to 30 years, and they're generally expected to last at least 20. Check out our article on how long TPO and PVC roofs last for more details.

They generally run $650-1000 per square. It can be tough to find someone who will do these for a good deal, many contractors we talk to charge over $1000 per square. However, when getting a flat roof done make sure you're getting good work done and not just picking the cheapest price without verifying the quality of work!

Last note on membranes is that in colder climates they are strongly preferred by contractors and sometimes mandated.

Coatings

Besides foam, most coatings are not commonly used in residential applications. However, in the right situation they can provide the longest warranty at the cheapest price!

Why aren't coatings more commonly used? I think there are 3 main reasons:

• Contractors are less familiar with them - Many roofers prefer materials they know well

• Aesthetics are different than more residential roofs - The look may not match traditional roofing

• Can't fix hidden damage - Because you're not tearing off the roof, you can't fix rotted wood that's underneath it. This makes it a poor choice for damaged roofs.

When to Consider a Coating

For these reasons we recommend the following approach when considering a coating for your flat roof:

1. Make sure you're fine with the aesthetics. Here are some pictures:

2. If your roof is in poor condition or has had leaks, do not get a coating. Here are some examples of rotted wood can be found underneath roofs:

But if your roof isn't in rough shape and you like the way a coating looks, you can get the best warranty at the lowest cost!! That alone makes it well worth considering. Check out our comprehensive overview of flat roofing materials to see differences between types of coatings.

Comparing the Options

This table tells a pretty clear story to help you make your choice. Modified bitumen is the cheapest, and if you need to save money it is still a fine way to do your roof. However in the long term a membrane will likely provide a lower total cost of ownership: you can get 30% more lifetime for just a 15% increase in cost. And if the the situation works our where a coating makes sense for you, you can get a roof that's going to last a long time for a very low cost!

The Bottom Line

Modified Bitumen is the most affordable option and a solid choice if budget is your primary concern.

Membranes offer the best long-term value - 30% more lifespan for just 15% more cost makes them a smart investment.

Coatings can provide exceptional value, but only if your roof is in good condition and you're comfortable with the aesthetics.

Important note on workmanship: Workmanship is always the most important factor in preventing leaks. This is even more true in flat roofing, where water has more opportunity to find and penetrate any vulnerability. While picking the right material type will get you longevity, picking the right contractor will help you avoid headaches.